This invention relates to protective relaying apparatus of the type which phase compares the current quantities at the two end portions of a protected transmission line section and is particularly useful in the segregated phase comparing apparatus shown and described in U.S. Pat. Application Ser. No. 402,687, filed Oct. 2, 1973 (as a continuation of now abandoned application Ser. No. 295,031, filed Oct. 4, 1972) now U.S. Pat. No. 3,893,008, dated July 1, 1975. Other U.S. patent applications disclosing similar segregated phase comparing relaying apparatus are Ser. No. 382,791, filed July 26, 1973, now U.S. Pat. No. 3,882,361, dated May 6, 1975; Ser. No. 387,070, filed Aug. 9, 1973, now U.S. Pat. 3,832,601, dated Aug. 27, 1974; Ser. No. 392,691, filed August 29, 1973, now U.S. Pat. No. 3,898,531, dated Aug. 5, 1975; and Ser. No. 436,758, filed Jan. 25, 1974; now U.S. Pat. No. 3,832,601, dated Aug. 27, 1974. Normally, when a protected line section is faulted, fault current will flow inwardly to the fault from the energized busses. Under these conditions, the direction of the current at each end of the internally faulted line section will be the same. However, under certain internal fault conditions as, for example, in the case of a high resistance line-to-ground or line-to-line fault, this is not always true. For example, assuming a strong power supply bus at one end and a weak power supply bus supplying a load at the other end, it is possible to have a net "outfeed" from the line section into the weak bus so that the relaying apparatus would not recognize the internal fault and the opening of the breakers would have to be accomplished by back-up or secondary protection.
In accordance with the teachings of this invention, the offsetting of the keying threshold from the zero line of the current wave, permits the phase comparing apparatus to recognize an internal fault even though there is an "outfeed" into one of the busses.